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Temperatures below 2.5 K have been reached in a continuously operating closed cycle cooler that has been engineered for space applications. This cooler is a modification of a 4 K development model cryocooler that consists of a two-stage Stirling cycle precooler with a closed cycle Joule-Thomson (JT) stage. The Stirling cycle precooler and the compressors for the JT system incorporate all the long life features of the Oxford/RAL single stage coolers which are now in orbit and operating successfully. The precooler now incorporates larger pistons and a slightly modified regenerator; this has led to a lower precooler temperature and increased JT cooling power. A new heat exchanger between the lower and upper stages of the precooler has reduced the heat load on the precooler. 4He is used in the JT system for temperatures around 4 K and 3He is used below 3 K. Different JT expansion orifices are used to optimise the cooler at 4 K and 2.5 K.

If a cryocooler connected to an instrument fails there is a parasitic load on the instrument due to conduction down the cold finger. In normal operation the cryocooler provides enough cooling to absorb this parasitic load in addition to the cooling required for the instrument. If multiple coolers are used on an instrument it is helpful if the failed cooler can be thermally disconnected to reduce the thermal load on the remaining coolers. Also, it may be a requirement that a “spare” redundant cooler be thermally connected to the instrument to provide cooling in place of a failed cooler. For both of these operations some type of thermal switch is required. The design and development of such a switch is described in this paper. When two metal surfaces are pressed together the contact between them is on the tips of irregularities on their surfaces; these asperities are plastically deformed as the load increases so that the area of contact is proportional to the load.